Field of the Invention
The present invention relates to plugged honeycomb structures. More particularly the present invention relates to a plugged honeycomb structure with high heat capacity, low initial pressure loss, small increase in pressure loss during the deposition of particulate matter, and high trapping efficiency of particulate matter.
Description of the Related Art
A plugged honeycomb structure is widely used as a honeycomb filter to trap particulate matter included in dust containing fluid, such as exhaust gas emitted from a diesel engine, or as a catalyst carrier to load catalyst for purification of harmful substance (e.g., NOx) in exhaust gas.
When a plugged honeycomb structure is used as a honeycomb filter, the honeycomb filter has to be regenerated by combusting particulate matter deposited at the honeycomb filter for removal on a regular basis. However, if a honeycomb filter is regenerated frequently, the fuel consumption thereof deteriorates, and so the regeneration frequency of a honeycomb filter has to be reduced. To reduce the regeneration frequency of a honeycomb filter, the amount of particulate matter to be combusted for removal at one time has to be increased. Meanwhile, if a large amount of particulate matter is combusted for removal at one time, internal temperature of the honeycomb filter rises, and the honeycomb filter may break. Then, there is a method of lowering the porosity of the partition wall of a honeycomb filter to increase the heat capacity of the honeycomb filter, and so an increase in the temperature of the inside of the honeycomb filter is suppressed even when a large amount of particulate matter is combusted for removal at one time. When the partition wall of a honeycomb filter has lowered porosity to increase the heat capacity of the honeycomb filter, however, increase in pressure loss during deposition of particulate matter increases, and the fuel consumption of an engine or the like may deteriorate.
In order to suppress such increase in pressure loss during deposition of particulate matter, there is proposed a filter that includes a trapping layer to trap particulate matter at the surface of the partition wall of the honeycomb filter to suppress penetration of particulate matter into the partition wall (e.g., see Non Patent Document 1).
According to a conventional method for manufacturing a honeycomb structure including a trapping layer, slurry to form the trapping layer (trapping-layer forming raw material) is applied to the porous partition wall (partition wall base material), followed by firing to form the trapping layer. Examples of the method for applying slurry to form the trapping layer includes immersion of a honeycomb structure in the slurry to form the trapping layer (trapping-layer forming raw material) or pouring the slurry to form the trapping layer into cells of the honeycomb structure. Then, when a porous membrane having a pore diameter smaller than that of the partition wall and having thickness thinner than that of the partition wall is formed at the surface of the porous partition wall, ceramic particles making up the porous membrane have to have a particle diameter that is smaller than the pore diameter of the partition wall. The thus obtained honeycomb structure, however, has a problem that the slurry to form the trapping layer penetrates into pores of the partition wall (partition wall base material) of the honeycomb structure, and so initial pressure loss when letting exhaust gas pass through the obtained honeycomb structure increases.
Meanwhile there is another honeycomb structure that is prepared by filling pores of a porous supporting member with “substance capable of being removed later” to block these pores, followed by application of slurry including ceramic particles with a small particle diameter to the surface of the porous supporting member (e.g., see Patent Documents 1 to 3). Such “substance capable of being removed later” includes combustible substance (Patent Document 1), for example. When combustible substance is used, such combustible substance can be combusted for removal through a firing step performed later. Examples of the “substance capable of being removed later” include water and alcohol (Patent Documents 2 and 3). When water or alcohol is used, drying is performed after the application of slurry, whereby such water or alcohol can be removed.
[Patent Document 1] JP-A-H01-274815
[Patent Document 2] JP-B-S63-66566
[Patent Document 3] JP-A-2000-288324
[Non Patent Document 1] SAE Technical Paper 2008-01-0618, Society of Automotive Engineers (2008)